1. Field of the Invention
The present invention relates to a switch element, a memory element and a magnetoresistive effect element which use carbon nanotube.
2. Description of the Related Art
Simply, the most notable feature of carbon nanotube is a cylinder made of graphite having a hollow portion. The carbon nanotube can be roughly divided into a single-walled carbon nanotube (CNT; SWCNT) of a one bar shape, and a multi-walled carbon nanotube (Multi-walled CNT; MWCNT) having a concentrically nested structure. Basically, a great difference between them is a difference in diameter, and the diameter of the single-walled CNT is several nm or less, and the diameter of the multi-walled CNT is several nm to several dozens nm. An inner diameter of the multi-walled CNT is generally about ⅓ of the outer diameter, but as the inner diameter is closer to that of the single-walled CNT, a ratio of the inner diameter to the outer diameter is closer to 1.
The feature of the single-walled CNT is that a CNT having metallic property and a CNT having semiconductor property are present depending on a difference in the structure. Further, as to the single-walled CNT, several experiments have been conducted in which a band gap is controlled and single-walled CNTs having different electric property are jointed so that a transistor of a nanoscale is realized.
On the other hand, the multi-walled CNT basically has metallic property. Carbon nanotubes generally-have small influence of scattering due to the presence of defects and impurities in the carbon nanotube, and its electric resistance is lower than that of copper. For this reason, the carbon nanotubes are expected also as interconnection with nm diameter whose resistance is low.
As one application of the carbon nanotubes mentioned above, the application to electronic devices is very effective. However, a serious problem which should be solved in order to realize this application is the following two points.
The first is the problem of contact between a carbon nanotube and metal which is necessary for wiring from an element manufactured by the carbon nanotube. Normally when the metal is allowed to adhere to the carbon nanotube by the vacuum evaporation method, contact resistance therebetween becomes very high. When the element itself becomes small, the contact resistance conceals the resistance of the entire element, and thus the element property is significantly deteriorated. For this reason, it is one of industrially very important problems to select metal which realizes the satisfactory contact with carbon nanotubes.
The second is that even if a single electronic device manufactured by carbon nanotubes is realized, it is an important problem to realize mass-production of carbon nanotubes in order to integrate them. In the present situation, a main method of manufacturing a carbon nanotube includes an arc discharge method and a chemical vapor deposition (CVD) method.
In the arc discharge method, simply, carbon rods used for an electrode are vaporized by discharge, and a carbon nanotube is taken out from collected soot. In this method, generally a very pure carbon nanotube can be taken out, but the step of replacing a carbon rod is required, and thus this method is not suitable for the mass production.
On the other hand, the CVD method is a method of manufacturing carbon nanotube by decomposing hydrocarbon gas over a metal catalyst (Co, Ni, Fe or the like). In this method, the purity is inferior to that in the arc discharge method, but this is excellent from the viewpoint of mass production.
In the present situation, however, neither a method of selectively growing a carbon nanotube having a specific structure nor a method of selectively taking out only a carbon nanotube having a specific structure from manufactured carbon nanotubes are developed. Therefore, this constitutes a large barrior to the mass production of transistors using single-walled CNTs. That is, in order to industrially apply electronic devices using carbon nanotubes, it is necessary to solve the manufacturing problems mentioned above or formulate a device which does not have to select a carbon nanotube having a specific structure.
Meanwhile, an attention is paid to carbon nanotubes due to their high mechanical strength, and it is known that its Young's modulus is about 1 TPa. For this reason, carbon nanotubes are tried to be utilized as probes of scanning probe microscopes by utilizing their strength. Recently, memory elements which utilize the strength of the carbon nanotube are proposed as described in Thomas Rueckes, et al., “Carbon Nanotube-Based Nonvolatile Random Access Memory for Molecular Computing”, Science, vol. 289, 94-97, 7 Jul. 2000”.
In this memory element, one-bit element is basically composed of a structure that two electrodes sandwich a bundle of carbon nanotubes. In principle, the bundle of the carbon nanotubes is brought into contact with any one of electrodes (electrode 1) by an electrostatic attracting force which is generated by applying a voltage between the carbon nanotubes and the electrode 1. This state is maintained by their van der Waals force so that a switching operation having a memory function is realized.
Further, when a voltage is applied between the other electrode (electrode 2) and the carbon nanotubes, the electrode 1 is separated from the bundle of the carbon nanotubes by electrostatic attracting force at this time, so that the memory can be erased. When resistance between the electrode 1 and the carbon nanotubes is measured, the state can be read. It goes without saying that the element utilizes high endurance (mechanical strength) with respect to the repeating operation of contact and separation of the carbon nanotubes. Basically, since a bundle of carbon nanotubes having arbitrary structure may be manufactured, the problem of the mass production mentioned above can be avoided.
When, however, the electrode 1 is formed on a substrate and a bundle of the carbon nanotubes is provided thereon, it is necessary to form the electrode 2 so as to float three-dimensionally, and this is very difficult. When one-bit element is tried to be small in order to achieve high integration, the bundle of the carbon nanotubes should be small. In such a situation, it is necessary to make the bundle of the carbon nanotubes uniform in all the elements, but it is very difficult to realize this state using the current technique in a nano-level. Further, a driving voltage for contact with the electrodes rises due to the strength of the carbon nanotubes, and this causes deterioration in the element property itself.